In recent years, medical diagnostic devices employing photon counting of radial rays typified by a Single Photon Emission Computed Tomography (SPECT: gamma camera) and a Positron Emission Tomography (PET) have been increasingly introduced. In the photon counting of radial rays, a detector is required to have high time resolution and also perform detection of the energy intensity of each single-photon radial ray, and count filtering according to the energy intensity is carried out.
For example, a slight amount of a gamma-ray source, such as technetium, is introduced into the inside of the living body, and then a gamma-ray source distribution in the living body is determined from position information of gamma rays to be emitted, whereby associated diseases, such as a blood-flow state in the living body and ischemia, are diagnosed. For the detection, a SPECT (gamma camera) device is used and a scintillator and a photomultiplier are usually used as a detector of gamma rays.
The basic configuration of the SPECT device is introduced in FIG. 25 and the like, for example, in a prior art of Patent Literature 1 (JP 2006-242958A). A specific example of signal processing for determining the incident position and the energy intensity of the gamma rays entering the detector is described in Patent Literature 2 (JP 2006-508344T), for example.
FIG. 1 is a view for explaining the outline of gamma ray detection. In the gamma ray detection, when a gamma ray 2 generated from a gamma-ray source 1 in the living body enters a scintillator 4 passing through a collimator 3, the scintillator 4 exhibits a fluorescence, and then the fluorescence is detected by photomultipliers 5 disposed in the shape of an array. The photomultipliers 5 amplify the same to produce current pulses, and then the current pulses are output to an arithmetic unit 7 as an incident light quantity value to each photodetection element through a converter 6 containing a voltage converter, an amplifier, and an A/D converter.
On the other hand, a gamma ray 8 which undergoes Compton scattering in the living body to be attenuated passes the collimator 3, and then detected in some cases. The signal is a noise which has lost the original position information. Or, a noise emitted as an unusually high signal caused by cosmic rays or the like is mentioned. The SPECT device filters these noises by energy discrimination from primary gamma rays which are not subjected to scattering. The arithmetic unit 7 performs the noise discrimination and the position determination of each gamma ray based on an output from the converter 6 connected to each photomultiplier. When the scintillator 4 is formed with a solid plate, the light emission is simultaneously detected by the plurality of photomultipliers 5. The arithmetic unit 7 specifies the gamma ray energy from the total output and specifies the incident position of the gamma rays from the center of gravity of the output, for example. In order to determine each gamma ray incidence as an independent event, these operation need to be performed at very high speed. Thus, the number of times of events of the gamma rays which are determined to be a primary (not a noise) is counted, and then the gamma-ray source distribution in the living body is identified.
The photon counting of radial rays involving such energy discrimination has been adopted also for X-ray transmission imaging in addition to transmission imaging using gamma rays having high transmission ability in recent years, and the effect has been increasingly recognized. For example, Patent Literature 3 (JP 2011-24773A) and Patent Literature 4 (JP 2004-77132A) each describe one example of such devices, and the application thereof to a mammography and an X-ray Computed Tomography (CT) has been expected. However, since the frequency of the incident radial rays is high in the case of X-rays, the photon counting has been demanded to have time resolution higher than that of gamma rays for medical treatment.
On the other hand, in Patent Literature 5 (JP 2011-97581A), the applicant suggests a new image pickup device by photon counting in which the dynamic range is increased using time division and screen division by a plurality of pixels in combination while following the circuit configuration of a Complementary Metal Oxide Semiconductor (CMOS) imager. Such a device can also be used as a device for photon counting in which the entire pixel array in a chip is one light receiving surface.
When detecting radial rays using such a semiconductor image pickup device and a scintillator, the number of photons emitted from the scintillator with the incidence of the radial rays reflects the energy intensity of the radial rays. By performing photon counting thereof, the detection sensitivity comparable to that of a photomultiplier can be achieved. However, the scintillation light is subjected to the photon counting herein and the number of photons of the radial rays is not counted.